Long before Taylor Swift became a superstar for writing songs about her self-absorbed exes, Martha’s Vineyarder Carly Simon wrote “You’re So Vain” about . . . well . . . someone (your guess is as good as ours). On Saturday, the legendary Simon joined Swift onstage at Gillette Stadium for a surprise performance of the song in front of about 55,000 Swift fans. Swift tweeted to fans after the local show was over, “Got to sing ‘You’re So Vain’ with Carly Simon at Gillette Stadium tonight!!!!! Unreal. I could not possibly look up to her more.” Swift has previously performed with Simon’s ex James Taylor; the two were at Tanglewood together just last summer. Swift also recently performed with Mick Jagger, who sang backing vocals on the original recording of “You’re So Vain.” Shortly after appearing with Swift at Gillette, Simon tweeted, “A resounding Hallelujah! I love Mick #mickjagger.” (Jagger celebrated his 70th birthday this weekend.) Swift’s own breakup songs are rumored to be about Taylor Lautner, Joe Jonas, John Mayer, and Jake Gyllenhaal (who was spotted on the Vineyard last week), but, like Simon, she never quite confirms.